Inner ears are found in all vertebrates, with substantial variations in the form and function of their sensory organs. Each animal has two inner ears, one on each side of its head.

Ear overview, in context

In mammals, the outer ear focuses and directs sound waves into the middle ear. In the middle ear, the energy of these pressure waves is translated into mechanical vibrations of the middle ear’s bone structure. The cochlea of the inner ear propagates these mechanical signals as waves in fluid and membranes, and finally transduces them to nerve impulses which are transmitted to the brain.

The vestibular system of the inner ear is responsible for the sensations of balance and motion. It uses the same kinds of fluids and detection cells (hair cells) as the cochlea uses, and sends information to the brain about the attitude, rotation, and linear motion of the head. The type of motion or attitude detected by a hair cell depends on its associated mechanical structures, such as the curved tube of a semicircular canal or the calcium carbonate crystals (otolith) of the saccule and utricle.

The inner ear is innervated by the eighth cranial nerve in all vertebrates.

Non-humans

Birds have an auditory system similar to that of mammals, including an outer ear, middle ear, and cochlea, though their middle ear has only one bone whereas the evolution of mammalian auditory ossicles resulted in three bones. Reptiles, amphibians, and fish do not have cochleas but hear with simpler auditory organs or vestibular organs, which generally detect lower-frequency sounds than the cochlea.